Proton transfer energy barrier

For the intermolecular proton transfer in the dimers, similar behavior and structures as in the previously described pyrrolo-pyrroles are found, being the nonconcerted TS the true states, with only one imaginary frequency, and Cs symmetry. The proton transfer energy barriers of the dimers range between 59... 

Fig. 3.5 a Reactant complex, b Intermediate complex, and c Product complex consisting of G" with NOj in presence of a water molecule. The water molecule facilitates proton transfer. Gibbs barrier above the arrows) and released below the arrows) energies (kcal/mol) at each step are given. A negative barrier energy implies a barrierless reaction [63]... 

Table 9.4. Barriers to intramolecular proton transfer (energy differences between the Q and C2V structures) in malonaldehyde calculated in different appproximations...

Proton transfers from strong acids to water and alcohols rank among the most rapid chemical processes and occur almost as fast as the molecules collide with one another Thus the height of the energy barrier the activation energy for proton transfer must be quite low... 

We have seen that 10" M s is about the fastest second-order rate constant that we might expect to measure this corresponds to a lifetime of about 10 " s at unit reactant concentration. Yet there is evidence, discussed by Grunwald, that certain proton transfers have lifetimes of the order 10 s. These ultrafast reactions are believed to take place via quantum mechanical tunneling through the energy barrier. This phenomenon will only be significant for very small particles, such as protons and electrons. 

Many computational studies in heterocyclic chemistry deal with proton transfer reactions between different tautomeric structures. Activation energies of these reactions obtained from quantum chemical calculations need further corrections, since tunneling effects may lower the effective barriers considerably. These effects can either be estimated by simple models or computed more precisely via the determination of the transmission coefficients within the framework of variational transition state calculations [92CPC235, 93JA2408]. 

This is one of the simplest systems for which isomers can exist. Ab initio calculations of the energetics for this system have shown that HCN+ is 17.7 kcal mol-1 less stable than HNC+ and has an energy barrier to isomerization of 30.9 kcal mol-1. Even though this barrier is substantial, isomerization occurs in the reactions with CO and C02, in parallel with a small proton transfer channel,23 viz.,... 

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